Capillary transport zones have been provided to convey (drops of patient sample along a path to a test area such as is provided by an ion-selective electrode,) and/or to a drop of reference liquid to form, e.g., an electrically conductive interface. Examples are shown in U.S. Pat. Nos. 4,233,029 and in 4,310,399. In the first of these, the opposing surfaces that are spaced apart a capillary distance are held together, with such a spacing, by means of adhesive, column 11, lines 1-6. In the second of these two, the surfaces are said to be joined with their capillary spacing, by the use of ultrasonic bonding. To permit such bonding, plastics are preferred.
Highly preferred plastics are those that are readily manufacturable and provide adequate support when used in a test element containing such a capillary transport zone. The problem has been that the material of choice, relative to these manufacturing considerations, is polystyrene, which has a serious disadvantage: it is not readily wetted by the patient samples of choice. Thus, polystyrene typically forms a high equilibrium contact angle with water and serum, specifically, 87.degree. and 83.degree., respectively, for a typical polystyrene. Such poor wettability tends to make the flow behavior of patient sample through the transport zone, erratic and unpredictable. Although geometric surfaces on the polystyrene can be used to overcome such erratic behavior, a more convenient construction of the transport zone would be one in which the surface is inherently more wettable. In such a case, the need for geometric surface designs would be avoided. Prior to this invention, it has been difficult to find a plastic that is both more wettable and has the manufacturability of plastics like polystyrene.
Although wetting agents have been applied to the polystyrene in an effort to solve the wettability problem, these agents in turn tend to have the disadvantage of interacting with the patient sample in one way or another. For example, a physical interaction of swelling occurs when using gelatin as the wetting agent as described in my U.S. Pat. No. 4,549,952 issued Oct. 29, 1985. This swelling has an advantageous function of increasing the viscosity of the flowing liquid, as noted in the patent. However, it also requires careful spacing tolerances, lest the gelatin swell to the point of preventing necessary liquid flow, e.g., as described in column 7, lines 11-16.